Typically, the main portion of a rocker arm (excluding a roller) is integrally made using a lost wax process or a hot molding process. These processes provide enough strength to the rocker arm, but are expensive.
Some rocker arms are mass-produced by press-molding a metal blank. This type of rocker arms is manufactured in such a manner that a metal plate is firstly blanked into the developed form of a rocker arm body having an opening in its center for mounting a roller; both sides of the opening being outwardly expanded as shown in FIGS. 34a-1, a-2 and a-3, then the expanded portions of both sides of the opening are downwardly folded to form side walls of the rocker arm body so as that the cross-section is an inverted U shape as shown in FIGS. 34b-1, b-2, and one end of the side walls are draw-processed and joined to each other to form a valve stem receiving portion. Further, downwardly pressing the joined end of the side walls while upwardly drawing the side walls to form a roller inserting portion as shown in FIGS. 34c-1, c-2 and c-3, then working the other end of the side walls into a shape corresponding to the pivot of the play adjuster to form a pivot engageable portion as shown in FIG. 34d-1, d-2 and d-3, and further making a hole on the outer walls of the roller inserting portion to provide a roller-supporting portion at which the roller is to be fixed with an axle.
However, in such a roller mounting rocker arm as described above, it is difficult to assure the precise working of both the valve stem receiving portion and the roller inserting portion, since these portions are integrally shaped by drawing and pressing the rocker arm body both upwardly and downwardly because of its form with inverted U-shaped cross-section, which process is rather more similar to cold-forming than one-piece metal molding. Further, the thickness of the rocker arm body is reduced during a cold-forming process with its complex steps, which results in decreased rigidity of the rocker arm.
Therefore, it is an object of the present invention to provide an improved rocker arm having enough rigidity and a high degree of freedom in design.
Further, simplification of each of the steps of the manufacturing process is desired, and so it is another object of the invention to provide a method for manufacturing a rocker arm which realizes an increase of productivity.
It is also desired to design a rocker arm so as to prevent interference with the moving elements and concentration of stress. And it is an another object of the invention to provide a rocker arm which can avoid stress concentration and has higher durability.
It is further an object of the invention to provide a roller mounting rocker arm which realizes a reduction of the processing cost.
It is still further an object of the invention to provide a method for manufacturing a metal-processed rocker arm body having a high rigidity and durability.
It is also an another object of the invention to provide a method for manufacturing a roller mounting rocker arm having high preciseness and being suitable for mass-production.